There is both commercial and military interest in generating very high power laser beams which can be focused to near the diffraction limit. Recent advances in high power fiber amplifiers have made available single power outputs of approximately one to two kilowatts with near pristine beams. However, power scaling interest extends to much higher power outputs, which requires a combination of many (e.g., 10's-100's or more) such fiber amplifier outputs into a single beam, while preserving their diffraction limited focusing property (e.g., beam quality or BQ). One conventional system uses a diffractive optical element (DOE) to enable the coherent combination of the output of many such fiber amplifiers. The coherent beam combination (CBC) requires active phasing of the outputs of the fiber amplifiers. Also, another conventional system uses an incoherent method where spectral beam combination (SBC) has been proposed and employed.
Although the use of a 2D fiber array offers the promise of a more compact CBC system, the large number of fibers in a single 2D array leads to much more complex controls of phase, and other optical properties such as polarization and optical path length, as well as the challenges associated with alignment precision of integration of the large number of free-space fiber outputs with the beam combining optical elements. One approach to mitigate the complexity of coherently combining a large number of fiber amplifiers is via a two-stage coherent combination scheme. This allows separate and independent control loops to mitigate fiber count and signal-to-noise requirements in any given control loop. Another conventional approach makes use of a hybrid coherent DOE and incoherent spectral beam combining (SBC). The SBC does not require phase control, but both of these approaches (CBC and hybrid CBC/SBC) still require a large integration of a complex 2D array of beams with high alignment precision.